1. Field of the Invention
The present invention relates to the field of diode-pumped, solid-state laser systems, and more specifically, it relates to an improvement in the design of such systems to achieve an increase in the efficiency of energy extraction from such systems.
2. Description of Related Art
Advances in laser diode technology have resulted in multi-element diode bars capable of producing in excess of 80 Watts of continuous laser output. These advances have resulted in an explosion of diode-pumped solid-state lasers. Until very recently, diode-pumped solid-state lasers were classified as either end-pumped or side-pumped. End-pumped refers simply to those designs wherein the output of the laser diode pump source is coupled into the end of a laser rod [see FIG. 3]. The diode pump source 302 can be either a single emitter or an array. The diode output is relayed to the end of the laser rod 304 by either lenses (e.g., cylindrical lens 306 and spherical lens 308), fibers, mirrors or lens ducts. With an appropriate choice of coupling optics, the diode radiation can be concentrated principally into the lowest order, TEM00, mode. Another choice of coupling optics can be designed to fill the end of the rod completely with the diode radiation channeled through the rod by total internal reflection from the rod barrel. Many of these designs employ lens ducts 402 to direct the diode radiation into the end of the laser rod 304 [as shown in FIG. 4]. End-pumped lasers are routinely available but high beam quality versions are limited to relatively low average power output.
Another common geometry for diode-pumped solid-state lasers is side pumping. In this configuration [as shown in FIGS. 5A and 5B] the diode light enters the laser medium through the barrel. There are numerous embodiments of this basic design: close-coupled, cavity filled and directed. In the close-coupled embodiment, the diodes (e.g., diode bars 502) are placed in close proximity to the laser medium (laser rod 504). Also, a cooling jacket 506 may surround the laser rod 504. In this configuration, the majority of the diode radiation enters the laser medium in spite of the large divergence of the diode light. The diode-pump radiation is distributed throughout the rod 504 but is often non-uniform due to the exponential absorption of the pump radiation.
Often side-pumped lasers will employ coupling optics 602 and a cavity surrounding the laser rod [as shown in FIG. 6]. In this cavity-filled embodiment, the diode laser radiation can make several passes through the laser medium (e.g., laser rod 504) since it is continually reflected by the reflector and cooling jacket 604 of the surrounding cavity. In the directed embodiment, the intervening optics between the diodes (e.g., diode bars 502) and the laser medium are designed to concentrate the diode radiation inside the laser rod 504. In this design, the diode-pump radiation typically makes a single-pass through the laser rod. Side-pumped laser designs offer a degree of simplicity but typically exhibit a low concentration of diode-pump radiation in the TEM00 mode relative to end-pumped lasers. This is a result of the fact that the pumping radiation enters through the barrel of the rod thereby concentrating a large amount of energy around the periphery of the rod. Efficient energy extraction occurs principally from the interior of the laser medium. Hence, side-pumped geometries typically provide a lower energy extraction efficiency than end-pumped systems.
Several side-pumped designs attempt to overcome the limitation on extraction efficiency by reflecting the laser beam 702 off of the pump faces. Two of the most common designs are shown in FIGS. 7 and 8. In the design of FIG. 7, the beam 702 makes a zig-zag path through the laser medium 704 which is side-pumped by a conventional diode array 706. The side opposite the pump face is coated with a high reflective coating 708 to reflect the laser light back towards the pump surface. Once the prescribed number of reflections have occurred, the laser beam 702 strikes a region of the medium 704 which is anti-reflection coated to allow passage of the beam out of the crystal. Other embodiments of this design utilize fiber coupling of the diode radiation to the surface of the laser medium.
Another design which utilizes internal reflections of the laser beam 802 is shown in FIG. 8. In this design, the diode-pump radiation 706 is directed through optics 806 (e.g., a cylindrical lens 810 and a half wave plate 812) and enters a polished face of the laser medium 808 in a typical side-pumped geometry. The laser beam 802 makes a single, grazing incidence, total internal reflection off of the pump face 804. By grazing off of the pump surface, higher extraction efficiency can be achieved. However, this design is subject to significant beam aberration resulting from the thermal stress and distortion of the pump face.
Yet another design relying on total internal reflection is directed towards achieving a monolithic, diode-pumped unidirectional ring laser such as described in U.S. Pat. No. 4,578,793, of Kane et al., entitled SOLID STATE NON-PLANAR INTERNALLY REFLECTIVE RING LASER, which is incorporated herein by reference. In this design a non-planar ring laser is formed by polishing the reflecting surfaces out of plane [as shown in FIG. 9]. The beam 902 enters the base of the laser medium 904 where it refracts towards the first out of plane face 906. The beam reflects from this surface towards the top of the laser crystal. The beam reflects from the top surface towards the second out-of plane face 908 where it reflects again towards the entrance surface. The laser medium 904 is pumped from one surface 910. This design was introduced to provide a monolithic ring laser for unidirectional operation containing a polarizer, a half-wave plate and a Faraday Rotator. By placing this laser crystal inside a magnetic field, all three elements can be embodied in a single crystal. The out-of-plane reflections are enabling features of this design for achieving unidirectional, narrow frequency output. This design produces excellent single-frequency, unidirectional performance at low average power. These and similar designs are limited to low power operation due to thermally induced birefringence when the pump power is greater than approximately 100 W.